pack-objects: fix off-by-one in delta-island tree-depth computation

pack-objects: fix off-by-one in delta-island tree-depth computation

When delta-islands are in use, we need to record the deepest path at
which we find each tree and blob. Our loop to do so counts slashes, so
"foo" is depth 0, "foo/bar" is depth 1, and so on.

However, this neglects root trees, which are represented by the empty
string. Those also have depth 0, but are at a layer above "foo". Thus,
"foo" should be 1, "foo/bar" at 2, and so on. We use this depth to
topo-sort the trees in resolve_tree_islands(). As a result, we may fail
to visit a root tree before the sub-trees it contains, and therefore not
correctly pass down the island marks.

That in turn could lead to missing some delta opportunities (objects are
in the same island, but we didn't realize it) or creating unwanted
cross-island deltas (one object is in an island another isn't, but we
don't realize). In practice, it seems to have only a small effect.  Some
experiments on the real-world git/git fork network at GitHub showed an
improvement of only 0.14% in the resulting clone size.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>